1. Field of the Invention
The present invention relates to heat dissipation from a reticle in an extreme ultra violet lithography tool.
2. Background Art
In advanced optical lithography, a master pattern on a substrate, referred to as a reticle, is copied onto a wafer by projecting deep ultra violet (DUV) light through, or reflecting extreme ultra violet (EUV) light off of the reticle, and then passing that light through imaging optics on to a wafer. The requirement to copy on to the wafer sub-micron features requires the wafer and reticle be held in a precise relationship with respect to the imaging optics. In the case of a DUV step and scan lithography tool the reticle is usually held to a reticle stage by vacuum, but for an EUV tool, where a vacuum is required for EUV transmission, electrostatic force is used in place of vacuum. The mechanical interface between the reticle and its stage is called a chuck.
In general, the reticle is heated by the exposure illumination and as a result the reticle is distorted. For most DUV tools the reticle substrate is low expansion fused silica and the temperature rise of the reticle is limited by heat transfer mechanisms enhanced by the atmosphere so that the resulting distortion is kept to acceptable levels. For a EUV tool, however, where features smaller than 30 nm will be imaged, and where the heat load on the reticle is higher, thermal distortion is a greater problem. EUV reticles are typically near zero expansion materials such as Ultra Low Expansion (ULE) glass. Even with this advantage, given the vacuum environment, and low infrared emissivity of the reticle, thermal distortion requires more attention to cooling.
In a typical EUV chuck design the reticle is held by electrostatic force against the pimpled surface of an electrostatic clamp. Gas may be injected between the reticle and clamp to promote good heat transfer from the reticle to the reticle stage. Further cooling fluid may be circulated in passages in the chuck behind the electrostatic clamp to limit temperature rise and resultant distortion. The electrostatic clamp is attached to a rigid, near zero expansion structure that also holds the metrology references for the reticle stage positioning system.
To achieve the required accuracy the reticle stage may incorporate magnetic levitation to allow ultra precise control of the reticle position. The magnetically levitated portion of the reticle stage is referred to as the short stroke stage and is made up of a low expansion stiff structure, force actuators, metrology references, and the reticle chuck. The short stroke module reacts against a long stroke stage that moves along the scanning axis. The short stroke stage can be designed with an integral chuck, or a separate chuck module the can be removed from the short stroke structure. In either case, it is highly desirable not to run liquid on to the short stroke because the vibration introduced by the fluid flow and the dynamic characteristics of the connecting hoses reduce pattern positioning accuracy. Cooling fluids are run to the long stroke stage to cool actuator coils and because of the high degree of isolation provided by magnetic levitation cause no significant problem.
FIG. 1 shows a conventional system 100 including a short stroke stage portion 102. The short stroke stage portion 102 includes a reticle 104 and a thin front plate 106 of a chuck (a combination of elements 108 and 112), the two being separated by a gas gap 106. Water flows through a heat removal section 110 positioned “below” front plate 108. Below heat removal section 110 there is a stiff structure 112 of the chuck.
Reticle 104 is placed on a reticle stage that includes the short stroke stage (interchangeably referred to as short stroke stage or SS throughout) for fine positioning and a long stroke stage (not shown in this figure) (interchangeably referred to as long stroke stage or LS throughout) for coarse positioning. Typically, these stages are connected together and at least one is cooled using a flowing fluid. In most cases, optics are used between the reticle and the wafer to reduce the pattern before exposure and to compensate for any distortion of the pattern. Exposure is done for a series of patterns to fabricate the various layers used to form electrical devices on the wafers. In order for the fabricated devices to work properly the patterns must be overlaid with only slight variations from ideal. This is even more important in extreme ultra violet (EUV) systems because the characteristic dimensions of the devices being fabricated can be on a scale of less than 50 nanometers (nm). Even a shift of less than 10 nm from a desired to an actual position of each layer of the patterns can render all the devices on a wafer unusable.
In lithographic systems, light used for exposure can be absorbed by optical elements as well as by ambient gas. To reduce the absorption effects in EUV, the exposure process is performed in vacuum and a reflective reticle is used. An EUV reticle typically reflects less than about 60 percent of the incident light. The interaction of the light with the reticle (also called the actinic heat load) raises the temperature of the reticle, which can distort the patterns on the reticle, and thus the copied patterns on the wafers. This distortion is more troublesome in a EUV system because, as discussed above, there is very little tolerance allowed during the copying of the patterns. Also, although other systems that operate at longer wavelengths than EUV use transmissive reticles, which are far less sensitive to distortion, they cannot be used with EUV light due to complete absorption.
Cooling systems have been developed to compensate for increases in reticle temperature caused by exposure to the light. Typical cooling systems run a liquid through channels in the long and/or short stroke stage to keep the stages cool, which also cools the reticle on the short stroke stage. Unfortunately, the cooling systems vibrate the stages because of the constant flow of liquid. The vibrations can blur patterns being exposed on the wafer. Even if the cooling system is moved to the long stroke stage, vibration may still be transferred through physical connections between the long stroke stage and the short stroke stage.
Therefore, what is needed is a system and method of controlling thermal distortion of the reticle in an EUV tool without utilizing a liquid cooled reticle chuck.
Therefore, what is also needed is a system and method for providing enough surface area on a radiative coupling device of a short stroke stage to allow for effective radiative heat transfer from a reticle and through the short stroke stage. Also, what is needed is a cooling system and method for a short stroke stage holding a reticle that does not require any fluid flow through the short stroke stage and that does not require the short stroke stage to be connected to the long stroke stage to reduce unwanted vibrations.